Ion selector



A. O. C. NIER ION SELECTOR Nov. 24, 1953 Filed April 12. 1948 5 Sheets-Sheet l ALFRED NIER www Nov. 24, 1953 Filed April l2. 1948 COLLECTOR CURRENT A. o. c. NIER 2,660,677

ION SELECTOR 3 Sheets-Sheet 2 COLLECTOR CURRENT FIGA- v ,www

Nov. 24, 1953 A. o. c. NIER 2,660,677

ION SELECTOR Filed April 12, 1948 5 Sheets-Sheet 5 Jam L 7 JE3 J| IJV 6 REGULATOR POWER SUPPLY l H lGH VOLTAGE SUPPLY IN V EN TOR AT ORNEY Patented Nov. 24, 1953 UNITED STAT ortica N. sELncToa Application April l2, 1948, Serial No. 20,393

(Cl. -25il-4L9) -6 Claims. l

The .present invention relates to y massI spectrometers and has for its objectto provide improvements in `a masscspectrorneter vto increase the accuracy or" the instrument.

Mass `spectrometers are frequently v.used to measure the amount of each component of different gases present in a gaseous mixture, a sample of which isfed to the gasanalyzer portion of the mass spectrometer. As is well known in the art, a mass spectrometer involves `an ion co'llecting plate and the ion current to this plate at any lgiven accelerating voltage at the ion source of the mass spectrometer is a measure of the quantity of such ions present in the gaseous mixture, and thus a measure of the amount of that particular gaseous component in the mixn ture. Naturally current at the collector plate resultingfrorn other causes, that is, other than the said ions-Would provide afalse reading. Such interering current may result from stray or sec.. ondary electrons. The secondary electrons are caused by fast-moving ions-strikingmolecules of gas, the 4collecting slit, this is, the slit which precedes the collector plate, walls of the tube, etc. Extraneous ion current which falsies the reading may also result from slow-moving lpositive ions, slow-moving positive ions resulting from these ions having cofllded withvother ions or molecules Within the evacuated space. Thisy lat ter result of course occurs more excessively when the vacuum conditions are less perfect than is ordinarily desired.

The primary object of my invention is to pro vide an improvement in the mass spectrometer which will eliminate the extraneous currents referred to above which tend to falsify the current reading at the collector plate.

More particularly, it is one of the objects of my invention in one form to eliminate extaneous currents'resultingfrom stray or secondary electrons by providing a relatively weak magnetic field parallel to the collector slit for tying up stray and secondary electrons in that vicinity.

`Another object of 4my invention is to eliminate extraneous falsifying currents at the collector plate by providing a positive field, that is, an electrode having a positive potential ahead of the collector plate, the potential thereon being sufiicient to reject positive ions not moving at the velocity of the particular ions which it is desired to collect.

Another object of the invention is to provide an improvement for eliminating ialsifying currents as described comprising the positive iicld ahead of the collector plate, as in the preceding object, and also negative elds, that is, electrodes having'negative potentials, both before and after the positive field, so as to reject stray electrons 2 which may exist ahead of the positive lield and to reject secondary electrons which may b given orf from the collector plate itself. l

'Another object is to eliminate falsifying` currents by providing ra band-pass ilterpreceding the collector plate comprising condenser plates having similar voltages of opposite polarityimpressed thereon, with a median plane between the plates at ground potential and aligned' with the collector slit so that the lter passes only ions a predetermined desiredenergy. Y

'lne particularA problems which my `inventionis designed to overcome, `the manner of `meeting these problems, and numerous additional lobjectives and advantages of my invention will become apparent'from the following detailed description and annexed drawings wherein, l t'ig. l is a cross sectional View of a spectrometer Fig. 2 is a view taken along line 2--2 ol Fig. 1.

l 51g. 3 is a diagrammatic view of the'dispersion in a mass spectrometer as illustrated by certain sample gases. l

YFig. 4 is a graph villustrating the masking effect that occurs in a mass spectrometer under conditions of poor vacuum giving rise to the existence of slowed ions in the Aevacuated space.

Fig. 5 is a graph illustrating the the results of the mass spectrometer under conditions vof moreperfect vacuum.

Fig. Gisa -diagrammatic showing of the collector of a mass spectrometer arranged in accordance ,with one form of my invention.

(Eig. *7 is a diagrammatic showingV of the collector and its associated electrodes'illustrating another formof my invention involving the use of the positive field, both preceded and followed by negative fields, ahead of the collector plate.

F1g;8 is a diagranunaticzshowing of the collector of a mass spectrometer illustrating another iorin of my invention wherein a magnetic field is utilized parallel tothe collector slit ,for rejecting secondary electrons.

Fig. 9 is a diagrammatic showing of another form of my invention whereina band-pass filter is used to intercept slow-moving ions and-'electrons aheadV of the collector. l

Fig. 10 isa diagrammatic showing of a further form of my invention.

Fig.4 11 is an enlarged view of the spectrometer source including power supplies. Y 4 4 Referring to lg. 1 of the drawings, the mass spectrometer tube is shown at i0 and involves'a glass envelope sealed to the copper analyzer tube l I. The tube surrounding the collector plate itself is designated at l 2, and it is made of nickel and suitably secured to the copper tubing portion. Portions surrounding the collector plate "are made of nickel to avoid evaporation of metal on the insulation supporting the collector lead. The gas to be analyzed is fed into the instrument through the gas inlet tube I3, as shown, and is pumped out through the copper pump lead as shown at I4. By feeding the gas directly into the source, the pressure there will be considerably higher than in the rest of the apparatus. This feature is desirable because high pressure in the source gives a greater number of ions, hence, more sensitivity, and lofw pressure in the rest of the apparatus gives better resolution. The various leads for connection to the electrodes Within the tubes are sealed to the glass envelope as shown at I5 and I6. The ionization chamber itself is illustrated at I'I and as may be seen on Fig. 2, the filament F is heated to give an electron beam which is received on plate or trap `T. This electron beam is kept accurately aligned by means of a magnet field produced by a magnet having pole faces as indicated at I8. The electron beam is immediately above the slit 21 in plate S which is a shield plate.

Ions formed by collision of the electrons with the gas molecules are drawn through the slit 2 in plate S and accelerated through a series of plates J1, J2, J3, J5 and G. (See Fig. 1l.) J1 and J2 are a split pair and permit one to bend the beam to one side or the other to compensate for imperfections in the alignment as Well as for the slight bending of the beam produced by the magnetic eld used for aligning the electron beam. The remainder of the plates form a lens which not only increases the intensity of the ion beam entering the magnetic analyzer, but also prevents a dropping oi of ion intensity for ali but the lowest energy ions. The plate marked G is grounded along with the magnetic analyzer housing to which it is tied.

Voltage on the chamber II is supplied by the high voltage source I, designated High Voltage Supply, across which is a voltage divider the slider of which is connected to the chamber I1. This source is also across voltage divider 2 so that a fraction of thisl voltage may be impressed on plate J5 which is connected to the slider of divider 2. A separate voltage supply is provided for plates J1, J2 and J2 as designated at 3 and indicated Regulator Power Supply. This source is across voltage divider 4, the slider of which connects to plate J2 so that a fraction of the Voltage may be applied to the latter plate. The source 3 is also across series connected resistors 5, 6 and l, with divider 8 in parallel with resistor 6. Plate J1 is connected to the mid-point of resistor 6 and plate J2 is connected to the slider of divider 8. As can readily be seen, by this circuit arrangement the resistors 5 and 1 which are variable may be adjusted to equally adjust the voltages on plates J1 and J2, and divider 8 may be adjusted to vary the voltage on plate J2 relative to that on plate J1.

The magnetic field associated with the tube for bending the ion beam, and that resulting in dispersion of the ions of various masses, is in trapezoidal form as indicated at 20, the pole faces being indicated by numeral I9 on Fig. 2. The collector plate is indicated at 2I and it has the usual plate 22 preceding it with a longitudinal slit therein. The lead from the collector plate is indicated at 23 extending through a glass insulator 26 and passing outwardly from the tube through a cylindrical shielding member 24, the latter member having a flexible coupling, consisting of bellows 25, connecting it to tube I2.

The operation of the mass spectrometer is in# dicated diagrammatically in Fig. 3. The gas is bombarded with electrons at the source and the resulting ions are focused into a beam which is directed into the aforesaid magnetic eld of trapezoidal form. The action of this field bends the path of the ions, the ions having the greatest mass being bent the least, and vice versa. There` fore. the spectrometer effects a separation of the ions of the gaseous mixture, the various componen't ions in the example of Fig. 3 being H2+, He+, H2O1L and N2+. The apparatus may be adjusted to direct the selected ion into the collecting means. This adjustment is effected by adjusting the voltage drop through which the ions are accelerated at the source, as demonstrated by the following equations:

(1) l/zmv2 (kinetic energy gained by ion)=eV (potential energy lost) (2) Hev (magnetic force on particle) =mv2r (centrifugal force) Eliminating (1J) from (1) and (2) gives: (3) (m/e)V=kr2H2 o=ve1ocity of ion m/e=molecular Weight of particle divided by electron units in charge on particle.

V=difference in potential (volts) through which the particle fell r=radius of curvature in magnetic eld H :strength of magnetic field between poles (gauss) lc=a constant According to Equation 3 for an ion of a given m/e, the voltage (V) may be adjusted to produce a desired radius of curvature (r), assuming that the strength of the magnetic field (H) is kept constant. In other words, the accelerating voltage may be used to direct a selected ion to the collecting means. This theory is simple, but as a practical matter there are difficulties in collecting the ions. The present invention has been devised in an attempt to overcome certain of these difficulties as has been pointed out in the aforegoing statement of objectives.

Equation 3, referred to hereinbefore, rests on the assumption that all ions of the same mass and charge move at the same velocity after they leave the ion source (1J was eliminated to derive Equation 3). This is actually the case only if the ions move in a high vacuum; collisions with other ions and molecules will slow down a measurable number of the ions in a relatively poor vacuum. Such slow-moving ions will not be collected at the collecting means at the proper Voltage V. Worse than that, they will be collected at some larger voltage V1, which is supposed to collect some lighter ion, and will therefore interfere with the reading for the latter. The tendency for the slow-moving ions to interfere with the reading increases markedly with an increase in pressure. In Fig. 4 the voltage drop at the source is plotted against the intensity of the ion current at the collecting plates. The nitrogen peak was made at approximately 43 volts on a certain spectrometer, whereas the helium peak was made at approximately 300 volts. The heights of these peaks indicate the relative quantity of helium and nitrogen present, and are well defined only at low pressures. As the pressure rises, more and more N2 ions occupy the cross hatched area of the graph; that is, they are slowed down considerably and finally result in the curve shown in broken lines at 9 (Fig. 4). At

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te Y. r fle it serves as ritial bei -a l' ed by battery il as m ig. sb s the arrangement a negative a positive 'fi f The device o it does not have a g at the G0 56. a d therefore ru .ss

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